1. Field of the Invention
The present invention relates to an electric double layer capacitor and in particular to a material for an electrode plate which is used in the electric double layer capacitor.
In the following specification and claims, it will be convenient to refer to specific steel and iron materials. There are a variety of numbers which identify the same specific iron and steel materials according to standards identified in various systems throughout the world, as set forth in the following table:
______________________________________ American Iron And Steel Japanese UNS Institute "A1S1" Standards ISO ______________________________________ S43000 430 SUS430 8,H4 (1) S30100 301 SUS301 14 ______________________________________
Where: "UNS" is a U.S. standard and "ISO" is international organization for standardizations.
Hence, for example, the same material is variously known as: S43000, 430, SUS 430, and 8,H4(1). It is also possible to show a material in a group of materials. For example, "SUS430 (JIS G 4305)" means that "SUS430" is a material in a group "JIS G 4305" of materials.
2. Description of the Related Art
As for a capacitor having a large capacitance while making use of an electric double layer, there has been known, as disclosed in, for instance, U.S. Pat. No. 3,536,963, the disclosure of which is hereby incorporated by reference herein, a capacitor having a large capacitance in which an electric double layer is formed by coming carbon powder in contact with an electrolyte.
Referring now to FIG. 1, there is shown a sectional view of a unitary electric double layer capacitor element (hereinafter referred to as "element") 1 and a stacked element 6. The element 1 has a pair of electronic conducting and ionic insulating plastic films 2, a pair of carbon paste electrodes 3 each comprising a mixture of activated carbon powder and dilute sulfuric acid and arranged between these two plastic films 2, an ionic conducting and electronic insulating porous separator 4 which is positioned for preventing the electric conduction between the carbon paste electrodes 3, and non-conductive rubber gaskets 5 for holding the carbon paste electrodes 3 and for isolating the electrodes 3 from the outside. Maximum permissible voltage of the element 1 depends on electrolytic voltage of dilute sulfuric acid used for an electrolyte and thus in order to obtain a desired maximum permissible voltage, a plurality of elements are in series stacked to form the stacked element 6. The stacked element 6 thus formed has internal resistance including contact resistance among activated carbon particles within the carbon paste electrodes 3 of the element 1, contact resistance between adjacent elements 1 and so on since the stacked element 6 comprises a multilayer body of the element 1 having the above-mentioned structure. The internal resistance depends on pressures applied to the stacked element 6 from its upper and lower sides. In order to stabilize the internal resistance, as shown in FIG. 2, it is necessary to apply pressures of P (about 20 kg/cm.sup.2) or more thereto. FIG. 2 is a graph showing dependency of internal resistance of a conventional stacked element on pressures. Values of internal resistance (.OMEGA.) vary depending on a shape and size of the stacked element but a relation between the internal resistance and the pressure shows a similar tendency as shown in FIG. 2. Thus, the internal resistance is stabilized by applying pressures of about 20 kg/cm.sup.2 or more to the stacked element.
Referring now to FIGS. 3A to 3D, there are shown a front view (FIG. 3A) of a conventional radial type electric double layer capacitor, an underside view (FIG. 3B) thereof, a cross-sectional view (FIG. 3C) taken along a line A--A of FIG. 3A and an oblique view thereof (FIG. 3D).
As shown in those drawings, a stacked element 10 is sandwiched between a first electrode plate 11 having a lead terminal 11a and a second electrode plate 12 having a lead terminal 12a, and the resulting assembly is placed in a mold and is controlled to a fixed size through the both electrode plates 11 and 12 by means of pressure pins (not shown) provided on the mold in order to stabilize internal resistance. Applied pressure is set to be of about 20 to 50 kg/cm.sup.2 considering variation in a thickness of the stacked element 10. While applying the pressure to the stacked element 10, molten thermoplastic resin PBT (polybutylene terephthalate) 13 is injected into the mold through a gate 14 for injecting the resin under injection pressure of about 50 kg/cm.sup.2, hardened under cooling to be molded and packaged. After molding, the pressure pins are removed to leave holes 15 in their places.
Referring now to FIGS. 4A to 4F, there are shown a top view (FIG. 4A), a front vies (FIG. 4B), an underside view (FIG. 4C) and a right side view (FIG. 4D) of a conventional chip type electric double layer capacitor, a cross-sectional view (FIG. 4E) taken along a line B--B of FIG. 4B, and an oblique view (FIG. 4F) thereof after molding.
As shown in those drawings, a stacked element 20 is sandwiched between a first electrode plate 21 having two lead terminal 21a at the same side and a second electrode plate 22 having a lead terminal 22a, and the resulting assembly is placed in a mold and is controlled to a fixed size through the both electrode plates 21 and 22 by means of pressure pins provided on the mold in order to stabilize internal resistance. Applied pressure is set to be of about 20 to 50 kg/cm.sup.2 considering variation in a thickness of the stacked element 20. While applying the pressure to the stacked element 20, molten heat-resistant PPS (polyphenylene sulfide) 23, which is a thermoplastic resin, is injected into the mold through a gate 24 for injecting the resin under injection pressure of about 80 kg/cm.sup.2, hardened under cooling to be molded and packaged. After molding, the pressure pins are removed to leave holes 25 in their places. After molding and packaging, the lead terminals 21a and 22a protruded from the resin package are subjected to a folding or bending process and are recessed in recess portions 23a for receiving the outer lead terminals so that the capacitor is easily surface-mounted on, for instance, a printed circuit board. The lead terminals after being folded are referred to as lead terminals 21b and 22b, respectively. The process for folding the lead terminals 21a and 22a and recessing the terminals in the recess portions 23a includes a step of strongly pushing the folded lead terminals on the resin package in order to control the thickness of the lead terminals protruded from the surface of the resin package to be of 0.15 mm or less. Thus, it is not practically admitted to control a thickness of the resin sandwiched between the lead terminals 21b and 22b and the first electrode plate 21 to be of 0.4 mm or less.
In the radial type electric double layer capacitor and the chip type electric double layer capacitor as shown in FIGS. 3A to 3D and 4A to 4F, respectively, a cold-rolled steel plate such as SPCC 0.3.sup.t (0.3 mm in a thickness) to 0.4.sup.t (0.4 mm in a thickness) as defined in JIS G 3141 was used as a material for the electrode plate since it is necessary to control deformation .delta. in the steel plate to be 0.2 mm or less even if the pressure (about 20 to 50 kg/cm.sup.2) for stabilizing the internal resistance and the pressure (about 50 kg/cm.sup.2 and about 80 kg/cm.sup.2) for injecting the respective resins are applied to the steel plate, as shown in FIG. 5A. FIG. 5A is a graph showing a relation between pressures (kg/cm.sup.2) applied by imitated pressure pin jigs and deformation .delta. (mm) in several steel plates due to the pressures. As shown in FIG. 5B, a steel plate 7 under test is placed between the stacked element 6 shown in FIG. 1 and an imitated mold 8 provided with imitated pressure pin jigs 9, and pressures are applied to the steel plate 7 from an upper side of the stacked element 6 by means of a pressurizer. Then, the steel plate 7 is withdrawn to measure the deformation .delta. in the steel plate as shown in FIG. 5C.
In association with a recent rapid advance in reduction of electronic equipments in size, particularly in reduction of portable equipments in size, demand for shrinking electronic parts and reducing a mounting area has been increased. In the conventional radial type and chip type electric double layer capacitors, SPCC (cold-rolled steel plate) has been used as a material for the electrode plate having the lead terminal. Thus, the thickness of the electrode plate must have fallen within the range of 0.3 to 0.4 mm. Now, thinning of the thickness of the molding resin causes an increase in pressure for injecting the resin into a mold, to thus be related to thickening of the electrode plate made of SPCC. Therefore, it was difficult to reduce the thickness of the completed electric double layer capacitor in terms of a package structure.